Antenna



W. SICHAK pri! 3, 1951 ANTENNA Filed Aug. 8, 1945 FIG.I

1N VEN TOR.

. WILLIAM SICHAK A TTORNEY Patented Apr. 3, 1951 ANTENNA William Sichak,Cambridge, Mass., assignor, by mesne assignments, to the United Statesof America as represented by the Secretary of War Application August 8,1945, Serial No. 609,652

Claims.

This invention relates to antennas for communication systems and isparticularly directed. to an antenna having means for shaping theprimary radiation pattern of a beam of electromagnetic energy emitted bya radiating element, such as a horn-type feed.

Such a radiating element has wide application. It can be used to match awave guide transmission line to free space and thus may be used forradiating waves of electromagnetic energy directly to free space.However, its principal a plication is in feeding energy to, andilluminating, a refiector adapted to direct the radiant energy into freespace in any desired beam pattern.

The gain, half power beam width and side lobes of the radiation patternof an antenna having a paraboloidal reflector depend upon the phase andamplitude distribution of the radiation pattern of the reflector feed.The latter may be referred to as the primary pattern of the feed. For acircular dish-like reflector the problem of feed is relatively simple.For a truncated paraboloidal reflector it is necessary to use a feedadapted to produce a beam suitably shaped for efficiently illuminatingthe reflector.

One objective in designing a paraboloid antenna is to realize themaximum possible gain consistent with side lobe requirements. In orderto get the highest gain, it is required to have a relatively uniformillumination across the aperture of the paraboloid and low spill overthe edge. To achieve this, one can either shape the primary beam of afeed to fit a given reflector or shape the reflector, such as to form atruncated par-aboloid, to fit a given feed, or both.

The gain of a paraboloid antenna with a given feed is a maximum if theamplitude at the edge is half the average amplitude of the illuminationover the aperture of the paraboloid. For a given feed, this provides asimple criterion for determining the siZe of the paraboloid. In general,the gain factor does not drop sharply with the increase of the apertureof the dish. Thus, a slightly bigger aperture can always be used toreduce the side lobes of the radiation pattern of the antenna. In mostcases, however, the converse problem is encountered; namely, for a givenparaboloid how to shape the primary pattern of a feed to achieve thebest results.

In order to design an antenna of high gain factor, it is necessary toshape a primary pattern of some special type to provide more uniformi1lu-- mination with little energy shooting outside the reflector. Thusit is frequently necessary that the radiation from a horn or a waveguide type feed meets certain requirements as to intensity near the edgeof the beam. For example, it may be desired that the intensity patternsin the H-plane and in the E-plane be similar. Another possiblerequirement would be that the radiation angle be about between the 10decibel points (points A and A in Fig. 1 with point B used as a basisfor comparison), as would be desired for illumination of a paraboloidfor which the focus is located in the plane of the periphery. Ordinarilythe radiation pattern for an open wave guide type feed is too broad inthe E-plane and too narrow in the H-plane to achieve the desiredillumination. By known means, the beam may be narrowed in the E-planeand adjusted so that the desired attenuation of the primary pattern isattained at the periphery of the'paraboloid or other suitably shapedreflector.

Accordingly, it is the principal object of the present invention toprovide means for broadening the primary radiation pattern of an openwave guide or horn type feed in the H-plane so that the attenuation inthe H-plane may be adjusted to be substantially the same as that in theE- plane, or vice versa.

Another object of the invention is to provide means for shaping theprimary pattern of a radiating feed element to fit specified reflectors.

Still another object of the invention is to provide a radiating feedelement for an antenna which is adapted to produce a desired energy beampattern.

Other objects, advantages and novel features of the invention willbecome apparent when taken into consideration with the accompanyingdrawings, in which:

Fig. 1 is a diagrammatic view illustrating the radiation patternordinarily produced by an open wave guide or horn type feed and thepattern desired according to this invention;

Fig. 2 is a top plan view of an open wave guide with a shaped endaccording to one embodiment of the invention; and

Fig. 3 is a perspective view of a horn type radiating element accordingto a modification of the invention.

Referring now to Fig. 2, l0 designates an open wave guide of rectangularcross-section and having a broader dimension a. Wave guide I 0 isadapted when properly excited, to radiate energy from'its open endor'aperturel l. 'When'iprop erly positioned relative to a reflector ofany suitable shape, wave guide I0 is adapted to feed energy to andilluminate the reflector. As indicated hereinbefore, it is frequentlydesirable that the primary radiation pattern from a wave guide or hornbe shaped to efliciently illuminate a specified reflector. According tothis invention, it is desired to broaden the primary pattern in theH-plane (the plane parallel to the magnetic vector H and perpendicularto the electric vector E). This is accomplished by Cutting ofi thecornersof the broad side walls of'wave guide iii as at l2 and 13 so thatthe broad side walls appear to be tapered inwardly and towards the freeen of Wave guide In. Thus in efiect the Wave guide aperture I l isincreased in one plane, or as illustrated in the H-plane.

The width of the primarybeaminthe'H-mlane is a function of both I) and:It has-been-found that with a constant angle the beam becomes broaderas b is increased, and .for a constant 5b, the beam becomes broader foran increasing 1;. A plot of the relationship between Wp/m (the db. powerwidth chosen as the representative angle .or beam width of interest inreflector illumination) of the vcut .corner Wave :guide 1-D .and theangle of cut shows that Wp/lO increases as qb is increased with ageneral maximum .at =45. For a larger o, the beam'becomesmore and moreflat with a practically constant "W 1o. Thus, theefiect of cutting the.corners as .at I2 and :13 is that more diffraction of the .beam isproduced and the primary beam in the H-plane from that normally producedby an openwave guide as indicated by the solid line configuration inFig. 1 is broadened-to that shown by the dotted line configuration.

From studies ,of the secondary patterns inside a reflector, particularlya truncated paraboloidal reflector, fed by a cut corner wave guide asI'll, there is shown a substantial advantage in .using this type of feedover the ordinary open wave With both the ,corners ,of wave guide In outoff at equal angles of as at I2 and [3, it will be apparent that thebeam will be :broadened symmetrically. I-n certaincases it may bedesired to obtain an asymmetrical primary beam pattern. This may beachieved by cutting off only one of the corners of wave guide it, or bycutting off both corners at unequal angles of .41.

A modification of :the present invention is shown in Fig. 3 as appliedto a horn-type feed r d atin e e In ge e al. a :horn comprise a flaredportion attached ,or connected at its smallest dimensions to a hollowpipe wave guide. As shown, horn I5 has a wave guide portion I6 and aflared portion l1, both having a rectangular cross-section adapted tofeed energy waves in the appropriate mode, the E-vectoror E-plane beingindicated by the arrow at E. Flared portion if! comprises side walls l8and 1,9 as extensions in the same plane of the respective side walls ofwave guide portion 16, and upper and lower walls 20 and 2| formed at .anangle to the remaining parallel walls of wave guide portion [.6 atthroat 22. The dimension between upper and lower Walls 20 and 2] islarger at mouth 23 than at throat 22. Thus, horn I5 is shown as beingflared out in the E-plane and may be called an E-horn.

According to this modification, one .or both .of the corners of each ofupper and lower walls 20 and 21 are cut off at an angle o to broaden theradiation pattern Produced by horn in t 4 H-plane asymmetrically orsymmetrically respectively.

While there has been described what is at present considered thepreferred embodiments of the invention, it will be obvioustoithose'skilled in the art that various changes and modifications maybe made therein without departing from the invention.

What is claimed is:

1. A radiating device for feeding a parabolic reflector comprising arectangular hollow pipe .wave guide for propagating electromagneticenergy, said wave guide having a pair of walls parallel to the magneticfield vector of said elec trcmagnetic energy, sa-id Wave guide beingprovided with an open end for radiating said electromagnetic energy, andeach of said walls of said pair of walls parallel to said magnetic fieldvector having an extension beyond said open end in the direction ofradiation, said extensions being substantially in the form oftrapezoids.

2. A radiating device comprising a rectangularhollow pipe wave guide forpropagating electromagnetic energy, said wave guide being provided withan open end for radiating said electromagnetic energy, and each of thewider walls of said wave guide having an extension beyond said open endin the direction of radiation, said extensions being substantially inthe form of trapezoids.

3. A radiating device according to claim 1 'wherein the non-parallelsides of each of said trapezoidal extensions make an angle of 45 andrespectively with the parallel sides thereof.

4. A radiating device for feeding a parabolic reflector comprising arectangular hollow pipe 'wave guide for propagating electromagneticenergy, said wave guide being :provided with a flared open end forradiating said electromagnetic energy, said wave guide having at saidopen end a pair of walls parallelto the magnetic fleld vector of saidelectromagnetic energy, and each of ,said walls vof said'pairof'wallsparallel to said magnetic field vector :having an extension beyondsaid open end in the :direction of radiation, said extensions beingsubstantially in the form .oftrapezoids.

'5, A radiating device according to claim 4 in which the tnon-parallelsides of each of said trapezoidal extensions make an angle of 45 and 135respectively with the parallel sides thereof.

WILLIAM SICHAK.

.REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date Re. 23,003 Barrow May 25, 19482,234,293 Usselman Mar. 11, 1941 2,235,506 Schelkunoff Mar. 18, 19412,297,202 Dallenbach Sept. 29, 1942 2,369,808 Southworth Feb. 20, 19452,423,073 Willoughby June 24, 1947 2,429,640 Mieher et a1. Oct. 28, 1947FOREIGN PATENTS Number Country Date 878,830 France Feb. 4, 1943

